In many lasers, a light-emitting element is added as a dopant to a compound that serves as a crystalline or amorphous host. The characteristics of a solid-state laser depend on the dopant and crystalline compounds that are selected. Light emitting dopant materials include all the trivalent rare earth ions.
All laser materials have characteristic energy levels and transitions so that photons are emitted at certain transitions when electrons drop from excited states to lower states. Likewise, the materials absorb light at characteristic wavelengths when they are in the ground state or other low levels.
Light absorption can be at a narrow or broad range of wavelengths depending on the transitions involved. Laser operation at the holmium 5I6 to 5I7, and 5I7 to 5I8 transitions have been reported at near 2.9 μm and 2.1 μm in several different host crystals. Holmium has few absorption bands for pumping in the visible and near-IR (infrared). Consequently, laser performances using broad spectral emission pump sources have been poor except where additional sensitizer (codopant) ions have been used.
Thulium (Tm) sensitized holmium laser materials have proven to be quite useful but have several disadvantages. For example, the near energy resonance between the Tm 3F4 and Ho 5I7 results in incomplete energy transfer from the sensitizer ions. At room temperature with otherwise optimal densities of sensitizer, transfer of only approximately 60% of the stored excitation density to holmium ions can occur. This incomplete transfer proportionally increases the already high lasing thresholds associated with holmium. Further, the interaction of Tm 3F4 and Ho 5I7 metastable ions create a detrimental up conversion loss process that severely limits energy storage lifetimes and small signal gains. In addition, the short pulse performance of Tm/Ho lasers are limited by the up conversion loss process and the relatively slow (about 20 μsec) energy transfer from Tm 3F4 and Ho 5I7. Finally, the thermal loading of the Tm/Ho laser material is increased by the incomplete energy transfer and up conversion losses, thereby limiting the utility of such material for average power production. Other sensitizer co-dopants also create problems.
Lasers exist in the form of laser diodes, crystal lasers and gas lasers, and optical fiber lasers, which are all known in the art. Optical fibers used for optical fiber lasers may be an all glass fiber, a glass fiber with a doped core and a cladding, or combination thereof. In addition a photonic bandgap structure may be used. The core of the optical fiber is doped with a dopant, such as listed above, the ions of which are pumped using light energy from one of many types of sources that include flashlamps of the correct wavelength and line width, laser diode arrays, crystal lasers and other optical fiber lasers. In some situations, the pump energy may be absorbed by non-lasing energy levels of the ions. However, upon receipt of the energy at the non-lasing energy levels electrons at those energy levels are boosted to higher energy levels or “states”. However, the electrons can only stay at higher energy levels for a limited amount of time before giving up their extra energy and fall to lower energy levels in what is termed a “transition” and emit photons of light at known wavelengths.
The newest member of the optical fiber family used to make optical fiber lasers is a photonic crystal fiber. Photonic crystal fibers utilize diffraction as a means to guide light in a glass fiber. The material in which the light is guided, i.e., the core of the optical waveguide, can have a relatively low refractive index and thus a lower density.
A photonic crystal fiber is made up of a regular geometric array of generally parallel, axial passages formed along the length of a solid optical fiber. To create the photonic crystal fiber a plurality of rods or fibers are disposed parallel to and about a solid, central rod or fiber to form a bundle. Each of the plurality of silica fibers surrounding the central fiber has an axial passage formed through it along its entire length. A rare earth dopant is added to the central fiber to provide optical gain to the laser, and the dopant preferably has a radial profile selected to enhance the stability of lower order modes through the photonic crystal fiber. All the fibers are preferably formed of a transparent, low-loss, damage resistant material such as silica. The index of refraction of the plurality of silica fibers surrounding the central fiber is different than the index of refraction of the doped central fiber. In the preferred embodiment of the invention the dopant is Holmium and it is directly pumped using 1.9 micron radiation.
The bundle of fibers are processed to transform them into a sintered cluster in the form of a geometric array (e.g. hexagonal) that is known as a photonic crystal fiber. A reflective coating is deposited on an outer surface of the array to confine pumped light therein. Light used to pump the laser is injected into the fiber bundle from the side by focusing it through small holes in the reflective coating, or by reflecting it off transverse Bragg gratings written into a fiber pigtail coupled to the end of the photonic crystal fiber. The mode field diameter of the photonic crystal fiber is controlled by properly selecting the diameter and spacing of the passages in the fibers surrounding the central rod.
A reflective coating is deposited on the outer surface of the photonic crystal fiber bundle to confine pump light therein. The reflective coating is preferably a metallic material such as silver or aluminum. Alternatively, the reflective coating may be formed by encasing the photonic crystal fiber bundle in a sleeve of material having a different index of refraction than the individual fibers making up the composite photonic crystal fiber.
When a photonic crystal fiber is used as a laser, the mode is repeatedly reflected off of a dielectric coating disposed on both ends of the fiber that is highly reflective at the laser wavelength and minimally reflective at the pump wavelength. After the mode gains sufficient power, it leaks through one of the reflective coatings at the end of the fiber in a steady stream. To accomplish this, it is preferred to form one of the reflective end coatings with less than 100% reflectivity at the output wavelength of the laser.